In recent years, humanoid robots, particularly the walking of humanoid robots has been drawing attention of many researchers. The majority of researches regarding humanoid robot walking uses zero moment point (ZMP), and controls the ZMP to keep the ZMP inside the supporting polygon. In this approach, a humanoid robot and the surrounding environment of the robot are accurately modeled and differential equations are solved. However, the modeling becomes difficult if there is an unknown element. Moreover, since solving differential equation consumes time, it is difficult to perform real-time control.
Another approach does not use the ZMP. For example, one conventional technology makes use of a periodic motion of moveable parts of a robot and adjusts the phase of the periodic motion so that the posture of the robot remains stable (see Japanese Laid-open Patent Publication No. 2005-96068). Here, the movable parts indicate the legs and arms of a robot.
Moreover, development of other technologies are also underway. These technologies aim to efficiently control the robot so as to allow the humanoid robot to stably perform various motions, while eliminating the need of modeling a humanoid robot or the surrounding environment thereof.
The robot walking control includes feedback control based on the rotation angle of the robot. By reducing the fluctuation or overshooting of the ZMP attributed to the feedback control, it becomes possible to stabilize the walking of a robot. Therefore, reducing the feedback control as much as possible is a key to stable robot walking. Moreover, if the feedback control causes fluctuation or overshooting of the ZMP frequently, the motors used in the walking control get exhausted. Thus, reducing the feedback control as much as possible is also a key to reduction of the motor exhaustion.